Silicon-carbon composite material and lithium ion battery

Abstract

The invention discloses a silicon-carbon composite material and a lithium ion battery, and relates to the technical field of lithium ion battery negative electrode materials. The preparation method comprises the following steps: immersing a nitrogen-sulfur co-doped porous carbon material into a liquid silicon alloy melt to make a silicon alloy permeate into a porous carbon skeleton, electrolyzing in a molten salt electrolyte to obtain the nitrogen-sulfur co-doped porous carbon material uniformly loaded with nano silicon particles, and forming a fluorine / nitrogen gradient doped carbon coating layer on the surface of the nitrogen-sulfur co-doped porous carbon material. And thus, the silicon-carbon composite material is obtained. The silicon-carbon composite material prepared by the invention can be used as a negative electrode active material of a lithium ion battery, and has high first efficiency, high rate capability and excellent cycling stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, and more particularly to a silicon-carbon composite material and a lithium-ion battery. Background Technology

[0002] Silicon materials have high specific capacity and are considered an important development direction for next-generation lithium-ion battery anode materials. However, the huge volume effect (about 300%) of silicon during charge and discharge leads to structural damage and poor cycle stability, which seriously limits its commercial application.

[0003] To mitigate the volume effect of silicon materials and improve cycling stability, silicon-carbon composite materials are often prepared by combining silicon with carbon. Examples include loading silicon into a carbon framework, coating silicon surfaces with carbon, or combining both. While these methods have alleviated volume expansion and improved cycling stability to some extent, problems such as uneven silicon distribution and weak bonding between the carbon coating and the silicon core remain. These issues result in less than ideal first-efficiency performance and cycling stability, leaving significant room for improvement. Summary of the Invention

[0004] Based on the technical problems existing in the background technology, the present invention proposes a silicon-carbon composite material and a lithium-ion battery.

[0005] This invention proposes a silicon-carbon composite material with a core-shell structure. The core is a nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon, and the outer shell is a carbon coating layer with fluorine / nitrogen gradient doping. In the carbon coating layer, the fluorine content gradually increases from the inside to the outside, and the nitrogen content gradually decreases from the inside to the outside.

[0006] The preparation method of the silicon-carbon composite material includes the following steps:

[0007] S1. Dissolve the polymer carbon source and pore-forming agent in a solvent, then add a sulfur source and a nitrogen source to obtain a spinning solution; perform electrospinning on the spinning solution to obtain a composite fiber membrane;

[0008] S2. The composite fiber membrane is first pre-oxidized and then heated and carbonized to obtain a nitrogen-sulfur co-doped porous carbon material.

[0009] S3. Immerse the nitrogen-sulfur co-doped porous carbon material in a silicon alloy melt, remove it and cool it to obtain a nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy.

[0010] S4. Electrolyze the nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy in molten salt electrolyte to obtain nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon.

[0011] S5. The nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon is immersed in a polymer monomer solution including nitrogen-containing polymer monomers and fluorine-containing polymer monomers, first electropolymerized, and then heated and carbonized to obtain the final product.

[0012] This invention first synthesizes a nitrogen- and sulfur-doped porous polymer composite fiber membrane via electrospinning. Then, after pre-oxidation and carbonization, a nitrogen- and sulfur-doped porous carbon material is obtained. This material is then immersed in a liquid silicon alloy melt, allowing the silicon alloy to permeate and uniformly load onto the nitrogen- and sulfur-doped porous carbon material through capillary permeation. Next, electrolysis in a molten salt electrolyte causes the silicon alloy to undergo electrochemical reduction, forming nano-silicon particles that are uniformly deposited within the porous carbon material. This yields a nitrogen- and sulfur-doped porous carbon material with uniformly loaded nano-silicon particles. Finally, a nitrogen- and fluorine-containing polymer coating layer is electropolymerized and then heated for carbonization. The method forms a fluorine / nitrogen gradient doped carbon coating layer on its surface. This is because the electropolymerization activity of nitrogen-containing polymer monomers is higher than that of fluorine-containing polymer monomers. Under the action of an electric field, nitrogen-containing monomers preferentially undergo in-situ polymerization on the surface of porous carbon and nano-silicon to form a nitrogen-rich inner layer. As the electropolymerization process proceeds, fluorine-containing monomers gradually participate in polymerization and are deposited on the outer layer. Subsequently, during the carbonization process, nitrogen elements are stably embedded in the carbon skeleton, while fluorine elements tend to be enriched on the surface, thereby further strengthening the gradient distribution structure of gradually decreasing nitrogen content and gradually increasing fluorine content from the inside to the outside, thus obtaining a silicon-carbon composite material. This invention achieves uniform confined deposition of nano-silicon within the pores of carbon particles through capillary infiltration of silicon alloy melt and electrochemical reduction. This allows silicon nanoparticles to be uniformly integrated into the carbon material, effectively mitigating volume expansion and improving the material's cycle stability. Simultaneously, the nitrogen-sulfur co-doped porous carbon material serves as a framework, achieving stable connectivity with the nano-silicon through various interfacial bonds and interactions. Specifically, oxygen-containing functional groups on the carbon particle surface form Si-OC bonds with the silicon surface, nitrogen-doped sites further chemically anchor the nano-silicon by forming Si-N or Si-ON bonds, while sulfur-doped sites provide chemical anchoring through flexible weak bonds or... Interfacial interactions buffer the volumetric stress during lithium intercalation. Through the synergy of multiple effects, nano-silicon maintains a stable long-term distribution in porous carbon materials. The volume expansion and migration of nano-silicon are suppressed through the synergistic effect of spatial confinement and chemical anchoring. The nitrogen and fluorine co-doped functionally graded carbon coating induces the formation of an SEI film through interfacial regulation and reduces the consumption of active lithium during cycling. The synergy of the two not only further improves the cycling stability of the material and increases the first efficiency, but also the nitrogen and sulfur doped in the carbon framework and the gradient doped nitrogen and fluorine in the carbon coating can construct high-speed electron / ion transport channels, effectively improving the conductivity of the material and increasing the rate performance.

[0013] Preferably, the mass ratio of the polymer carbon source, pore-forming agent, sulfur source, and nitrogen source is 1:0.5~1:0.05~0.1:0.1~0.2.

[0014] Preferably, the solid content of the spinning solution is 10wt%~18wt%; more preferably, the solid content of the spinning solution is 12wt%~15wt%.

[0015] In S1, the solvent is a conventional choice, such as one or more of N,N-dimethylformamide (DMF), dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP).

[0016] In S1, the types of polymer carbon source, pore-forming agent, sulfur source, and nitrogen source are all conventionally selected.

[0017] Preferably, in S1, the polymer carbon source is at least one selected from polyacrylonitrile, polyvinylpyrrolidone, polyamide, polyacrylamide, polybenzimidazole, polyaniline, polypyrrole, cellulose and its derivatives, and lignin.

[0018] Preferably, the pore-forming agent is at least one of sodium lignosulfonate, lignosulfonate, alkali lignin, enzymatically hydrolyzed lignin, polyethylene glycol, polyvinyl alcohol, polymethyl methacrylate, starch, and sucrose.

[0019] Preferably, the sulfur source is at least one of thiourea, thioacetamide, and sulfur-containing amino acids.

[0020] Preferably, the nitrogen source is at least one selected from melamine, urea, dicyandiamide, dicyandiamide, guanidine compounds, polyethyleneimine, polyaniline, and polypyrrole.

[0021] In S2, the process parameters for pre-oxidation and heating carbonization are conventional selections and can be adjusted according to actual needs.

[0022] Preferably, in step S2, the pre-oxidation step includes: heating to 150~250°C and holding at that temperature for 1~2 hours; the pre-oxidation is carried out in an air atmosphere.

[0023] Preferably, in step S2, the heating carbonization step includes: heating to 700~900℃ and holding at that temperature for 2~4 hours; the heating carbonization is carried out in a protective gas atmosphere, wherein the protective gas is selected from at least one of nitrogen, argon, and helium.

[0024] Preferably, in step S2, a cooling step may be included between the pre-oxidation and the heating carbonization.

[0025] Preferably, in S2, the heating rate of the pre-oxidation is 1~3℃ / min.

[0026] Preferably, in S2, the heating rate for carbonization is 4~6℃ / min.

[0027] In S2, after heating and carbonization, conventional post-processing steps may also be included, such as cooling, crushing, and grinding.

[0028] Preferably, the specific surface area of ​​the nitrogen-sulfur co-doped porous carbon particles is 150~350 g / m². 2 The pore volume is 0.4~0.9cm. 3 / g.

[0029] Preferably, in step S3, the temperature of the silicon alloy melt is 650~750℃, and the immersion time is 2~4h. By controlling the appropriate temperature of the silicon alloy melt and the immersion time in the silicon alloy melt, the silicon alloy can be fully penetrated into the pores and surface of the nitrogen-sulfur co-doped porous carbon particles, thereby improving the uniformity of the material.

[0030] Preferably, in S3, the silicon alloy is selected from at least one of alkaline earth metal silicon alloys and alkali metal silicon alloys.

[0031] Preferably, in step S3, the silicon alloy is selected from silicon-magnesium alloys, silicon-calcium alloys, or combinations thereof. Selecting silicon alloys of the aforementioned specific types provides good solubility and uniform silicon nucleation, which is beneficial for suppressing silicon agglomeration. Furthermore, its synergistic effect with the MgCl2 molten salt system can improve the dispersion and uniformity of silicon particles within the carbon framework, thereby enhancing the material's performance.

[0032] Preferably, the silicon alloy contains 30% to 70% silicon by mass.

[0033] Preferably, S3 is carried out in a protective gas atmosphere, wherein the protective gas is selected from at least one of nitrogen, argon, and helium.

[0034] Preferably, in step S4, the specific steps of electrolyzing the nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy in a molten salt electrolyte include: using the nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy as the cathode and a high-purity graphite electrode or platinum electrode as the anode, placing it in a molten salt electrolyte, and electrolyzing for 1-3 hours at 700-780°C with a DC voltage of 2.5-3.5V applied between the cathode and anode. Controlling the temperature of the molten salt electrolyte at 700-780°C (below the SiC formation temperature of 900°C to avoid SiC formation) ensures that the silicon alloy is fully activated, the molten salt is completely melted, and silicon can diffuse rapidly, while avoiding excessively high temperatures that cause interface instability and silicon particle detachment. Controlling the appropriate electrolysis voltage and time can obtain uniformly deposited, uniformly sized nano-silicon particles. This is because if the voltage is too low, silicon nucleation is difficult; if the voltage is too high, silicon-magnesium co-deposition easily occurs, silicon agglomerates, and the interface is unstable; if the time is too short, the silicon loading is insufficient; if the time is too long, the silicon particles grow, leading to pore blockage.

[0035] Preferably, in S4, the electrolysis is carried out in a protective gas atmosphere, the protective gas being selected from argon or helium.

[0036] Preferably, in step S4, the molten salt electrolyte comprises MgCl2 and at least one alkali metal or alkaline earth metal chloride, wherein the mass percentage content of MgCl2 is 30% to 70%. By controlling the composition of the molten salt electrolyte, uniform silicon deposition and reaction can be facilitated, thereby improving the dispersion and uniformity of silicon particles in the carbon framework.

[0037] Preferably, the molten salt electrolyte is a mixed molten salt of MgCl2 and NaCl or a mixed molten salt of MgCl2, NaCl and CaCl2.

[0038] Preferably, when the molten salt electrolyte is a mixed molten salt of MgCl2 and NaCl, the mass percentage content of MgCl2 is 30%~70%, and the balance is NaCl.

[0039] Preferably, when the molten salt electrolyte is a mixed molten salt of MgCl2, NaCl and CaCl2, the mass percentage content of MgCl2 is 30%~70%, the mass percentage content of NaCl is 20%~50%, and the mass percentage content of CaCl2 is 10%~40%.

[0040] Preferably, in S4, the electrolysis process can be carried out in constant voltage or constant current mode.

[0041] Preferably, in step S5, the electropolymerization voltage is 1~3V and the time is 10~30min. Within this voltage range, nitrogen-containing monomers preferentially polymerize at the conductive carbon / silicon interface, while fluorine-containing monomers gradually accumulate in the outer layer, forming a gradient coating layer. This is because nitrogen-containing monomers with lower polymerization potentials are more easily deposited in the inner layer, while fluorine-containing monomers are deposited more in the outer layer. Within this time range, nitrogen-containing monomers rapidly deposit to form a nitrogen-rich inner layer, while fluorine-containing monomers gradually participate in the formation of the outer layer, thereby forming a fluorine / nitrogen gradient coating layer with fluorine content gradually increasing from the inside out and nitrogen content gradually decreasing from the inside out.

[0042] Preferably, in step S5, the total concentration of fluorinated polymer monomers and nitrogen-containing polymer monomers in the polymer monomer solution is 1 wt% to 5 wt%, wherein the mass ratio of fluorinated polymer monomers to nitrogen-containing polymer monomers is 1:0.5 to 3. By controlling the respective amounts of fluorinated polymer monomers and nitrogen-containing polymer monomers and their mass ratio, the structure of the nitrogen-fluorine co-doped functional gradient carbon coating layer formed after heating and carbonization can be optimized, thereby further improving the cycling performance and rate performance of the material.

[0043] In S5, the types of fluorinated polymer monomers and nitrogen-containing polymer monomers are conventionally selected.

[0044] Preferably, in S5, the fluoropolymer monomer is selected from at least one of vinylidene fluoride, tetrafluoroethylene, fluoromethacrylates, and fluorostyrene.

[0045] Preferably, in S5, the nitrogen-containing polymer monomer is selected from pyrrole, aniline, vinylpyridine, acrylonitrile, vinylimidazole, or a combination thereof.

[0046] Preferably, in step S5, the heating and carbonization step includes: heating to 400~600℃ and holding at that temperature for 2~4 hours. By controlling the temperature and time of heating and carbonization in step S5, nitrogen can be stably embedded in the carbon framework, while fluorine tends to accumulate on the surface, thereby further strengthening the gradient distribution structure where the nitrogen content gradually decreases and the fluorine content gradually increases from the inside out.

[0047] Preferably, in S5, the heating carbonization is carried out in a protective gas atmosphere, wherein the protective gas is selected from at least one of nitrogen, argon, and helium.

[0048] Preferably, in S5, the heating rate for carbonization is 1~3℃ / min.

[0049] In this invention, the solvent for the polymer monomer solution is conventionally selected, for example, it may be one or more of acetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0050] The present invention also proposes a silicon-carbon composite material, which is prepared by the aforementioned preparation method.

[0051] The present invention also proposes a lithium-ion battery, the lithium-ion battery comprising a negative electrode, a positive electrode and an electrolyte, wherein the negative electrode comprises a negative electrode active material, and the negative electrode active material comprises the aforementioned silicon-carbon composite material.

[0052] The beneficial effects of this invention are as follows:

[0053] This invention first immerses a nitrogen-sulfur co-doped porous carbon material in a liquid silicon alloy melt, allowing the silicon alloy to penetrate into the porous carbon framework. Then, it is electrolyzed in a molten salt electrolyte to obtain a nitrogen-sulfur co-doped porous carbon material uniformly loaded with nano-silicon particles. Next, a fluorine / nitrogen gradient-doped carbon coating layer is formed on its surface by electropolymerization followed by heating and carbonization, thus obtaining a silicon-carbon composite material. The silicon-carbon composite material prepared by this invention can be used as a negative electrode active material for lithium-ion batteries, exhibiting high initial efficiency, high rate performance, and excellent cycle stability. Detailed Implementation

[0054] The technical solution of the present invention will now be described in detail through specific embodiments.

[0055] Example 1

[0056] A method for preparing a silicon-carbon composite material, comprising the following specific steps:

[0057] S1. Polyacrylonitrile and sodium lignosulfonate are dissolved in DMF, and then thiourea and melamine are added to obtain a spinning solution with a solid content of 14 wt%, wherein the mass ratio of polyacrylonitrile, sodium lignosulfonate, thiourea and melamine is 1:0.8:0.08:0.15; the spinning solution is placed in an electrospinning device and electrospinned under the conditions of 15 kV voltage and 15 cm receiving distance to obtain a composite fiber membrane;

[0058] S2. The composite fiber membrane is first pre-oxidized in air at a heating rate of 2℃ / min to 200℃ and held for 1.5h, then cooled. The pre-oxidized composite fiber membrane is then transferred to a tube furnace and heated to 800℃ at a heating rate of 5℃ / min under an argon atmosphere, held for 3h for carbonization, and allowed to cool naturally. It is then pulverized and ground to obtain a nitrogen-sulfur co-doped porous carbon material with a specific surface area of ​​300 g / m². 2 The pore volume is 0.8cm. 3 / g;

[0059] S3. Immerse the nitrogen-sulfur co-doped porous carbon material in a silicon-magnesium alloy (Si: 50wt%, Mg: 50wt%) melt at 700℃, keep it at argon atmosphere for 3h, and cool it to room temperature to obtain the nitrogen-sulfur co-doped porous carbon material loaded with silicon-magnesium alloy.

[0060] S4. Using a nitrogen-sulfur co-doped porous carbon material loaded with silicon-magnesium alloy as the cathode and a graphite rod as the anode, the material is placed in a molten salt electrolyte and electrolyzed for 2 hours at 750°C with a DC voltage of 3.0V applied between the cathode and anode to obtain a nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon; wherein the molten salt electrolyte is obtained by mixing MgCl2 and NaCl in a mass ratio of 1:1.

[0061] S5. The nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon is immersed in a polymer monomer solution and electropolymerized at 2V for 20min to form a polymer coating layer. Then, under an argon atmosphere, the temperature is raised to 500℃ at a heating rate of 3℃ / min and held for 3h for heating carbonization to obtain the final product. The polymer monomer solution consists of the following components by mass percentage: vinylidene fluoride (VDF) 1.2%, pyrrole 1.8%, and acetonitrile solvent 97%.

[0062] Using the silicon-carbon composite material prepared above as the negative electrode active material, a negative electrode slurry was obtained by adding water in a mass ratio of negative electrode active material, conductive carbon black and polyacrylic acid binder of 0.93:0.03:0.04. The slurry was coated on the current collector copper foil and rolled to obtain the negative electrode sheet.

[0063] Positive electrode preparation: The positive electrode slurry is prepared by adding lithium iron phosphate, conductive carbon black and PVDF in NMP at a mass ratio of 0.95:0.02:0.03. The slurry is then coated onto carbon-coated aluminum foil and rolled to obtain the positive electrode sheet.

[0064] After assembling the negative electrode, positive electrode, and separator to obtain the battery cell, the electrolyte is injected to obtain a lithium-ion battery. The electrolyte composition is as follows: 1 mol / L LiPF6, 2 wt% LiODFB (lithium difluorooxalate borate), 2 wt% DTD (ethylene sulfate), and the balance is solvent, which is composed of EMC (ethyl methyl carbonate), EC (ethylene carbonate), and FEC (fluoroethylene carbonate) in a mass ratio of 65:25:8. The separator is a polypropylene membrane coated with aluminum oxide.

[0065] Example 2

[0066] A method for preparing a silicon-carbon composite material, comprising the following specific steps:

[0067] S1. Polyacrylonitrile and sodium lignosulfonate are dissolved in DMF, and then thiourea and melamine are added to obtain a spinning solution with a solid content of 14 wt%, wherein the mass ratio of polyacrylonitrile, sodium lignosulfonate, thiourea and melamine is 1:0.5:0.05:0.1; the spinning solution is placed in an electrospinning device and electrospinned under the conditions of 15 kV voltage and 15 cm receiving distance to obtain a composite fiber membrane;

[0068] S2. The composite fiber membrane is first pre-oxidized in air at a heating rate of 2℃ / min to 200℃ and held for 1.5h, then cooled. The pre-oxidized composite fiber membrane is then transferred to a tube furnace and heated to 800℃ at a heating rate of 5℃ / min under an argon atmosphere, held for 3h for carbonization, and allowed to cool naturally. It is then pulverized and ground to obtain a nitrogen-sulfur co-doped porous carbon material with a specific surface area of ​​300 g / m². 2 The pore volume is 0.8cm. 3 / g;

[0069] S3. Immerse the nitrogen-sulfur co-doped porous carbon material in a silicon-magnesium alloy (Si:60wt%, Mg:40wt%) melt at 700℃, keep it at argon atmosphere for 3h, and cool it to room temperature to obtain nitrogen-sulfur co-doped porous carbon material loaded with silicon-magnesium alloy.

[0070] S4. Using a nitrogen-sulfur co-doped porous carbon material loaded with silicon-magnesium alloy as the cathode and a graphite rod as the anode, the material is placed in a molten salt electrolyte and electrolyzed for 2 hours at 750°C with a DC voltage of 3.0V applied between the cathode and anode to obtain a nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon; wherein the molten salt electrolyte is obtained by mixing MgCl2 and NaCl in a mass ratio of 1:1.

[0071] S5. The nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon is immersed in a polymer monomer solution and electropolymerized at 1.5V for 30 minutes to form a polymer coating layer. Then, under an argon atmosphere, the temperature is raised to 500℃ at a heating rate of 3℃ / min and held for 3 hours for heating carbonization to obtain the final product. The polymer monomer solution consists of the following components by mass percentage: vinylidene fluoride (VDF) 1.2%, pyrrole 1.8%, and acetonitrile solvent 97%.

[0072] Using the silicon-carbon composite material prepared above as the negative electrode active material, a negative electrode sheet was prepared according to the method of Example 1 and assembled to obtain a lithium-ion battery.

[0073] Example 3

[0074] A method for preparing a silicon-carbon composite material, comprising the following specific steps:

[0075] S1. Polyacrylonitrile and sodium lignosulfonate are dissolved in DMF, and then thiourea and melamine are added to obtain a spinning solution with a solid content of 12wt%, wherein the mass ratio of polyacrylonitrile, sodium lignosulfonate, thiourea and melamine is 1:1:0.1:0.2; the spinning solution is placed in an electrospinning device and electrospinned under the conditions of 15 kV voltage and 15 cm receiving distance to obtain a composite fiber membrane;

[0076] S2. The composite fiber membrane is first pre-oxidized in air at a heating rate of 1℃ / min to 250℃ and held for 2 hours, then cooled. The pre-oxidized composite fiber membrane is then transferred to a tube furnace and heated to 900℃ at a heating rate of 6℃ / min under an argon atmosphere for 4 hours for carbonization. After natural cooling, it is then pulverized and ground to obtain a nitrogen-sulfur co-doped porous carbon material with a specific surface area of ​​350 g / m². 2 The pore volume is 0.9 cm. 3 / g;

[0077] S3. Immerse the nitrogen-sulfur co-doped porous carbon material in a silicon-calcium alloy (Si: 30wt%, Ca: 70wt%) melt at 750℃, keep it at the temperature for 3h in an argon atmosphere, and cool it to room temperature to obtain the nitrogen-sulfur co-doped porous carbon material loaded with silicon-calcium alloy.

[0078] S4. Using a nitrogen-sulfur co-doped porous carbon material loaded with silicon-calcium alloy as the cathode and a graphite rod as the anode, the material is placed in a molten salt electrolyte and electrolyzed for 3 hours at 750°C with a DC voltage of 3.5V applied between the cathode and anode to obtain a nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon. The molten salt electrolyte is obtained by mixing MgCl2, NaCl, and CaCl2 in a mass ratio of 1:0.6:0.4.

[0079] S5. The nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon is immersed in the polymer monomer solution and electropolymerized at 3V for 10min to form a polymer coating layer. Then, under an argon atmosphere, the temperature is raised to 400℃ at a heating rate of 1℃ / min and held for 2h for heating carbonization to obtain the product. The polymer monomer solution is composed of the following components by mass percentage: 2.5% fluorostyrene, 2.5% acrylonitrile, and 97% acetonitrile solvent.

[0080] Using the silicon-carbon composite material prepared above as the negative electrode active material, a negative electrode sheet was prepared according to the method of Example 1 and assembled to obtain a lithium-ion battery.

[0081] Comparative Example 1

[0082] Nano-silicon powder and graphite were weighed and mixed evenly at a mass ratio of 1:9 to serve as the negative electrode active material. A negative electrode sheet was prepared using the above-mentioned negative electrode active material according to the method of Example 1, and then assembled to obtain a lithium-ion battery.

[0083] Comparative Example 2

[0084] A method for preparing a silicon-carbon composite material, comprising the following specific steps:

[0085] S1. Polyacrylonitrile and sodium lignosulfonate are dissolved in DMF, and then melamine is added to obtain a spinning solution with a solid content of 14wt%, wherein the mass ratio of polyacrylonitrile, sodium lignosulfonate and melamine is 1:0.8:0.15; the spinning solution is placed in an electrospinning device and electrospinned under the conditions of 15 kV voltage and 15 cm receiving distance to obtain a composite fiber membrane;

[0086] S2. The composite fiber membrane is first pre-oxidized in air at a heating rate of 2℃ / min to 200℃ and held for 1.5h, then cooled. The pre-oxidized composite fiber membrane is then transferred to a tube furnace and heated to 800℃ at a heating rate of 5℃ / min under an argon atmosphere, held for 3h for carbonization, and allowed to cool naturally. It is then pulverized and ground to obtain nitrogen-doped porous carbon material with a specific surface area of ​​300 g / m². 2 The pore volume is 0.8cm. 3 / g;

[0087] S3. Immerse the nitrogen-doped porous carbon material in a silicon-magnesium alloy (Si: 50wt%, Mg: 50wt%) melt at 700℃, keep it at argon atmosphere for 3h, and cool it to room temperature to obtain the nitrogen-doped porous carbon material loaded with silicon-magnesium alloy.

[0088] S4. Using nitrogen-doped porous carbon material loaded with silicon-magnesium alloy as the cathode and graphite rod as the anode, the material is placed in a molten salt electrolyte and electrolyzed for 2 hours at 750°C with a DC voltage of 3.0V applied between the cathode and anode to obtain nitrogen-doped porous carbon material loaded with nano-silicon; wherein the molten salt electrolyte is obtained by mixing MgCl2 and NaCl in a mass ratio of 1:1.

[0089] S5. Nitrogen-doped porous carbon material loaded with nano-silicon is immersed in a polymer monomer solution and electropolymerized at 2V for 20min to form a polymer coating layer. Then, under an argon atmosphere, the temperature is raised to 500℃ at a heating rate of 3℃ / min and held for 3h for heating carbonization to obtain the final product. The polymer monomer solution consists of the following components by mass percentage: vinylidene fluoride (VDF) 1.2%, pyrrole 1.8%, and solvent acetonitrile 97%.

[0090] Using the silicon-carbon composite material obtained above as the negative electrode active material, a negative electrode sheet was prepared according to the method of Example 1 and assembled to obtain a lithium-ion battery.

[0091] Comparative Example 3

[0092] A method for preparing a silicon-carbon composite material, comprising the following specific steps:

[0093] S1. Polyacrylonitrile and sodium lignosulfonate are dissolved in DMF, and then thiourea and melamine are added to obtain a spinning solution with a solid content of 14 wt%, wherein the mass ratio of polyacrylonitrile, sodium lignosulfonate, thiourea and melamine is 1:0.8:0.08:0.15; the spinning solution is placed in an electrospinning device and electrospinned under the conditions of 15 kV voltage and 15 cm receiving distance to obtain a composite fiber membrane;

[0094] S2. The composite fiber membrane is first pre-oxidized in air at a heating rate of 2℃ / min to 200℃ and held for 1.5h, then cooled. The pre-oxidized composite fiber membrane is then transferred to a tube furnace and heated to 800℃ at a heating rate of 5℃ / min under an argon atmosphere, held for 3h for carbonization, and allowed to cool naturally. It is then pulverized and ground to obtain a nitrogen-sulfur co-doped porous carbon material with a specific surface area of ​​300 g / m². 2 The pore volume is 0.8cm. 3 / g;

[0095] S3. Immerse the nitrogen-sulfur co-doped porous carbon material in a silicon-magnesium alloy (Si: 50wt%, Mg: 50wt%) melt at 700℃, keep it at argon atmosphere for 3h, and cool it to room temperature to obtain the nitrogen-sulfur co-doped porous carbon material loaded with silicon-magnesium alloy.

[0096] S4. Using a nitrogen-sulfur co-doped porous carbon material loaded with silicon-magnesium alloy as the cathode and a graphite rod as the anode, the material is placed in a molten salt electrolyte and electrolyzed for 2 hours at 750°C with a DC voltage of 3.0V applied between the cathode and anode to obtain a nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon; wherein the molten salt electrolyte is obtained by mixing MgCl2 and NaCl in a mass ratio of 1:1.

[0097] S5. The nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon is immersed in the polymer monomer solution and electropolymerized at 2V for 10min to form a polymer coating layer. Then, under an argon atmosphere, the temperature is raised to 500℃ at a heating rate of 3℃ / min and held for 3h for heating carbonization to obtain the product. The polymer monomer solution is composed of the following components by mass percentage: pyrrole 3% and solvent acetonitrile 97%.

[0098] Using the silicon-carbon composite material obtained above as the negative electrode active material, a negative electrode sheet was prepared according to the method of Example 1 and assembled to obtain a lithium-ion battery.

[0099] Comparative Example 4

[0100] A method for preparing a silicon-carbon composite material, comprising the following specific steps:

[0101] S1. Polyacrylonitrile and sodium lignosulfonate are dissolved in DMF, and then thiourea and melamine are added to obtain a spinning solution with a solid content of 14 wt%, wherein the mass ratio of polyacrylonitrile, sodium lignosulfonate, thiourea and melamine is 1:0.8:0.08:0.15; the spinning solution is placed in an electrospinning device and electrospinned under the conditions of 15 kV voltage and 15 cm receiving distance to obtain a composite fiber membrane;

[0102] S2. The composite fiber membrane is first pre-oxidized in air at a heating rate of 2℃ / min to 200℃ and held for 1.5h, then cooled. The pre-oxidized composite fiber membrane is then transferred to a tube furnace and heated to 800℃ at a heating rate of 5℃ / min under an argon atmosphere, held for 3h for carbonization, and allowed to cool naturally. It is then pulverized and ground to obtain a nitrogen-sulfur co-doped porous carbon material with a specific surface area of ​​300 g / m². 2 The pore volume is 0.8cm. 3 / g;

[0103] S3. Mix the nitrogen-sulfur co-doped porous carbon material with nano-silica sol (particle size D50 is 15nm) evenly, and then vacuum dry to obtain nitrogen-sulfur co-doped porous carbon material loaded with nano-silica, wherein the mass ratio of nitrogen-sulfur co-doped porous carbon material to nano-silica is 9:1.

[0104] S4. Using a nitrogen-sulfur co-doped porous carbon material loaded with nano-silica as the cathode and a graphite rod as the anode, the material is placed in a molten salt electrolyte and electrolyzed for 4 hours at 850°C with a DC voltage of 1.8V applied between the cathode and anode to obtain a nitrogen-sulfur co-doped porous carbon material loaded with nano-silica. The molten salt electrolyte is obtained by mixing CaCl2 and NaCl in a mass ratio of 1:1.

[0105] S5. The nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon is immersed in a polymer monomer solution and electropolymerized at 2V for 20min to form a polymer coating layer. Then, under an argon atmosphere, the temperature is raised to 500℃ at a heating rate of 3℃ / min and held for 3h for heating carbonization to obtain the final product. The polymer monomer solution consists of the following components by mass percentage: vinylidene fluoride (VDF) 1.2%, pyrrole 1.8%, and acetonitrile solvent 97%.

[0106] Using the silicon-carbon composite material obtained above as the negative electrode active material, a negative electrode sheet was prepared according to the method of Example 1 and assembled to obtain a lithium-ion battery.

[0107] Test case

[0108] The lithium-ion batteries assembled in the above embodiments and comparative examples were tested for rate and cycle performance. The testing methods are as follows:

[0109] Cycle performance: The battery is charged to 3.65V at 1C constant current constant voltage (CC-CV), with a cutoff current of 0.05C during the constant voltage stage, and discharged to 2.5V at 1C.

[0110] Rate performance: The battery is charged to 3.65V at 1C constant current constant voltage (CC-CV), with a cutoff current of 0.05C during the constant voltage stage, and discharged to 2.5V at 1C, 2C, and 3C.

[0111] The test results are shown in Table 1:

[0112] Table 1

[0113]

[0114] As can be seen from the results in Table 1:

[0115] Compared with the conventional silicon-carbon anode material in Comparative Example 1, the silicon-carbon composite material of the present invention can significantly improve the first-cycle efficiency and cycle performance of the battery as an anode active material.

[0116] Compared with Comparative Examples 2 and 3, Comparative Example 2 did not dope sulfur in the porous carbon material, and Comparative Example 3 did not dope fluorine in the carbon coating layer. The first efficiency and cycle performance of the battery were far inferior to those of the examples. This is because nitrogen and sulfur doping can enhance the conductivity and silicon affinity of the carbon skeleton and form a strong interaction with the outer carbon coating layer. The sulfur doping sites can also buffer the volume stress during the lithium intercalation process through flexible weak bonds or interface interactions. The nitrogen and fluorine co-doped functional gradient carbon coating layer can induce the formation of SEI film through interface regulation and reduce the consumption of active lithium during cycling. Through the synergy of the nitrogen and sulfur doped carbon skeleton and the gradient doped nitrogen and fluorine coating layer, the first efficiency, rate performance and cycle stability of the material can be significantly improved. In Comparative Example 4, SiO2 particles were physically adsorbed onto the carbon skeleton and then electrolyzed and reduced in a molten salt electrolyte. After reduction, the interfacial bonding was weak, and the particles were easily peeled off after cycling. In addition, the SiO2 particles were large in size, making it difficult for them to enter the micropores and causing them to accumulate and block the pores. The distribution of silicon particles in the material was uneven. Furthermore, the particles were prone to agglomeration during the reduction process, forming large particles that damaged the carbon skeleton and resulting in poor performance.

[0117] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A silicon-carbon composite material, characterized in that, The silicon-carbon composite material has a core-shell structure, with the core being a nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon and the outer shell being a carbon coating layer with fluorine / nitrogen gradient doping; in the carbon coating layer, the fluorine content gradually increases from the inside to the outside, and the nitrogen content gradually decreases from the inside to the outside. The preparation method of the silicon-carbon composite material includes the following steps: S1. Dissolve the polymer carbon source and pore-forming agent in a solvent, then add a sulfur source and a nitrogen source to obtain a spinning solution; perform electrospinning on the spinning solution to obtain a composite fiber membrane; S2. The composite fiber membrane is first pre-oxidized and then heated and carbonized to obtain a nitrogen-sulfur co-doped porous carbon material. S3. Immerse the nitrogen-sulfur co-doped porous carbon material in a silicon alloy melt, remove it and cool it to obtain a nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy. S4. Electrolyze the nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy in molten salt electrolyte to obtain nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon. S5. The nitrogen-sulfur co-doped porous carbon material loaded with nano-silicon is immersed in a polymer monomer solution including nitrogen-containing polymer monomers and fluorine-containing polymer monomers, first electropolymerized, and then heated and carbonized to obtain the final product.

2. The silicon-carbon composite material according to claim 1, characterized in that, The mass ratio of the polymer carbon source, pore-forming agent, sulfur source, and nitrogen source is 1:0.5~1:0.05~0.1:0.1~0.

2.

3. The silicon-carbon composite material according to claim 1, characterized in that, The polymer carbon source is at least one of polyacrylonitrile, polyvinylpyrrolidone, polyamide, polyacrylamide, polybenzimidazole, polyaniline, polypyrrole, cellulose and its derivatives, and lignin. The pore-forming agent is at least one of sodium lignosulfonate, lignosulfonate, alkali lignin, enzymatically hydrolyzed lignin, polyethylene glycol, polyvinyl alcohol, polymethyl methacrylate, starch, and sucrose. The sulfur source is at least one of thiourea, thioacetamide, and sulfur-containing amino acids; The nitrogen source is at least one of melamine, urea, dicyandiamide, dicyandiamide, guanidine compounds, polyethyleneimine, polyaniline, and polypyrrole.

4. The silicon-carbon composite material according to claim 1, characterized in that, In S2, the pre-oxidation step includes: heating to 150~250℃ and holding at that temperature for 1~2 hours; the pre-oxidation is carried out in an air atmosphere; In S2, the heating carbonization step includes: heating to 700~900℃ and holding at that temperature for 2~4 hours; the heating carbonization is carried out in a protective gas atmosphere, wherein the protective gas is selected from at least one of nitrogen, argon, and helium.

5. The silicon-carbon composite material according to claim 1, characterized in that, In S3, the temperature of the silicon alloy melt is 650~750℃, and the immersion time is 2~4h; The silicon alloy is selected from silicon-magnesium alloy, silicon-calcium alloy, or a combination thereof; S3 is carried out in a protective atmosphere, wherein the protective gas is selected from at least one of nitrogen, argon, and helium.

6. The silicon-carbon composite material according to claim 1, characterized in that, In step S4, the specific steps for electrolyzing the nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy in a molten salt electrolyte include: using the nitrogen-sulfur co-doped porous carbon material loaded with silicon alloy as the cathode, and using a high-purity graphite electrode or platinum electrode as the anode, placing it in a molten salt electrolyte, and applying a DC voltage of 2.5~3.5V between the cathode and anode at 700~780℃ for 1~3 hours; the electrolysis is carried out in a protective gas atmosphere, and the protective gas is selected from argon or helium. In S4, the molten salt electrolyte includes MgCl2 and at least one alkali metal or alkaline earth metal chloride, wherein the mass percentage content of MgCl2 is 30% to 70%.

7. The silicon-carbon composite material according to claim 1, characterized in that, In S5, the total concentration of fluorinated polymer monomers and nitrogen-containing polymer monomers in the polymer monomer solution is 1wt%~5wt%, wherein the mass ratio of fluorinated polymer monomers to nitrogen-containing polymer monomers is 1:0.5~3.

8. The silicon-carbon composite material according to claim 1, characterized in that, In S5, the fluoropolymer monomer is selected from at least one of vinylidene fluoride, tetrafluoroethylene, fluoromethacrylate, and fluorostyrene. In S5, the nitrogen-containing polymer monomer is selected from at least one of pyrrole, aniline, vinylpyridine, acrylonitrile, and vinylimidazole.

9. The silicon-carbon composite material according to claim 1, characterized in that, In S5, the voltage of the electropolymerization is 1~3V, and the time is 10~30min; In S5, the heating and carbonization step includes: heating to 400~600℃ and holding at that temperature for 2~4 hours; The heating and carbonization is carried out in a protective gas atmosphere, which is selected from at least one of nitrogen, argon, and helium.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes a negative electrode, a positive electrode, a separator, and an electrolyte. The negative electrode includes a negative electrode active material, which includes a silicon-carbon composite material as described in any one of claims 1 to 9.

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